The amount of earth adhering to the roots of a tree when prostrated by the wind is, under favorable circumstances, very considerable, and upon their decay forms an oblong mound of greater or less magnitude, and a slight depression is left where the tree stood. These little hillocks are often by the inexperienced mistaken for Indian graves. From the paucity of these little "tree-mounds" we infer that no very great antiquity can be assigned to the dense forests of Wisconsin; for, during a long period of time, with no material change of climate, we would expect to find great numbers of these little monuments of ancient storms scattered everywhere over the ground. Whether the greater extent of treeless country in former times was owing to natural or artificial causes it is now difficult to determine, but the great extent of ancient works within the depths of the present forests would seem to indicate that the country was at least kept free from trees by the agency of man. Many of these tree-mounds were observed on and about the ancient works. Another curious circumstance that may be noticed by inspection of the figures of mounds accompanying this work is the gradual transition, as it were, or change of one form into another. Examples can be found of all forms, from a true circle through the oval and elongated oval to the oblong mounds and long ridges. Again, there is a succession of mounds, from the simple ridge of considerable size at one end and gradually diminishing to a point at the other, through the intermediate forms, having one, two, three, or four projections to the "turtle-form." In this way, also, we may trace a gradual development (so to speak) of nearly all the more complicated forms. It is not pretended to assert that this was the order in which the mounds were erected; or that the aborigines gradually acquired the art by successive essays or lessons. Indeed, we are led to believe that the more complicated forms are the most ancient. The relative ages of the different works in Wisconsin, so far as they can be ascertained from the facts now before us, are probably about as follows: First and oldest. The animal forms, and the great works at Aztalan. Second. The conical mounds built for sepulchral purposes, which come down to a very recent period. Third. The indications of garden-beds planted in regular geometrical figures or straight lines. Fourth. The plantations of the present tribes, who plant without system or regularity. Thus the taste for regular forms and arrangements, and the habits of construction with earthy materials seems to have been gradually lost, until all traces of them disappear in our modern degenerate red men.